AtmosphereThe layer of gases surrounding Earth, about 480 km thick, held in place by gravity. It contains 78% nitrogen, 21% oxygen, and trace gases.BarometerAn instrument that measures atmospheric pressure. Falling pressure usually indicates approaching storms; rising pressure suggests fair weather.HumidityThe amount of water vapour present in the air. Relative humidity is expressed as a percentage — 100% means the air is fully saturated.PrecipitationAny form of water falling from clouds to the ground — rain, snow, sleet, or hail. India receives most of its precipitation during the monsoon.CondensationThe process by which water vapour cools and changes from a gas into liquid water droplets, forming clouds, dew, and fog.EvaporationThe process by which liquid water changes into water vapour (gas) when heated. The sun's energy drives evaporation from oceans, lakes, and rivers.FrontThe boundary between two different air masses (warm and cold). Weather changes — clouds, rain, and wind shifts — typically occur along fronts.Pressure SystemA region of relatively high or low atmospheric pressure. Low pressure brings clouds and rain; high pressure brings clear skies.Jet StreamA narrow band of fast-moving air (200-400 km/h) high in the atmosphere. The subtropical jet stream plays a crucial role in driving India's monsoon.Greenhouse EffectThe trapping of heat by gases like CO₂ and methane in the atmosphere. Without it, Earth would be about 33°C colder — but excess greenhouse gases cause global warming.Dew PointThe temperature at which air becomes saturated and water vapour begins to condense into liquid. When air temperature equals dew point, fog or dew forms.Wind ChillThe perceived decrease in temperature felt by the body due to wind. Moving air strips heat from skin faster, making it feel colder than the actual temperature.IsobarA line on a weather map connecting points of equal atmospheric pressure. Closely spaced isobars indicate strong winds; widely spaced ones indicate calm conditions.TroposphereThe lowest layer of Earth's atmosphere (0-12 km) where all weather occurs. Temperature decreases with altitude at about 6.5°C per kilometre.AlbedoThe reflectivity of a surface. Fresh snow has high albedo (reflects 80-90% of sunlight), while dark ocean water has low albedo (reflects only 6%).ConvectionThe circular movement of air caused by heating. Warm air rises, cools, and sinks — this drives thunderstorms, sea breezes, and large-scale weather patterns.Coriolis EffectThe deflection of moving air due to Earth's rotation. It curves winds right in the Northern Hemisphere, creating the spin of cyclones and trade wind patterns.ITCZThe Intertropical Convergence Zone — a belt of low pressure near the equator where trade winds from both hemispheres meet. Its seasonal shift drives India's monsoon.Western DisturbanceEastward-moving weather systems originating in the Mediterranean that bring winter rainfall and snowfall to North India, crucial for the Rabi (winter) crop.Nor'westerViolent thunderstorms that strike eastern and northeastern India in April-May. Called Kalbaisakhi in Bengal, they bring heavy rain, hail, and gusty winds. 